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Old 11th May 2014, 10:11 PM   #1
maxw is offline maxw  United Kingdom
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Default How to interpret measurement spectrum graphs and peaks

I'm reasonably new to using to taking measurements and I was wondering if someone could help me out with some aspects of interpreting the results, particularly the spectrum graphs and their peaks.

First of all, why do a lot of graphs have a large peak in the middle? Arta does this but RMAA graphs don't. Eg:
Click the image to open in full size.

Some of my noise graphs in RMAA have a peak at 50hz, even when the mains isn't connected to my device. Any idea why?
Click the image to open in full size.

Sometimes I get large peaks at 50hz and then at 150/250/350/450 etc. What would cause this?
Click the image to open in full size.

Any advice appreciated!
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Old 11th May 2014, 10:48 PM   #2
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Quote:
Originally Posted by maxw View Post
Sometimes I get large peaks at 50hz and then at 150/250/350/450 etc. What would cause this?
This could be from a couple of things. Firstly it could be mains ripple from inadequate smoothing of the rectified supply. Secondly it could be leaked flux from a mains transformer coupling into signal wires.
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Old 11th May 2014, 11:26 PM   #3
davada is offline davada  Canada
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Quote:
Originally Posted by maxw View Post
I'm reasonably new to using to taking measurements and I was wondering if someone could help me out with some aspects of interpreting the results, particularly the spectrum graphs and their peaks.

First of all, why do a lot of graphs have a large peak in the middle? Arta does this but RMAA graphs don't. Eg:
Click the image to open in full size.

Some of my noise graphs in RMAA have a peak at 50hz, even when the mains isn't connected to my device. Any idea why?
Click the image to open in full size.

Sometimes I get large peaks at 50hz and then at 150/250/350/450 etc. What would cause this?
Click the image to open in full size.

Any advice appreciated!

In the first graph the large peak in the middle in the source fundamental. If you use sat a 1kHz tone then the fft will display this as a peak at 1kHz. Because the fundamental has the greatest power it will stand much taller than the other peaks. The other peaks are mostly harmonics of the fundamental tone at 2kHz, 3kHz, 4kHz and so on. If a very pure tone is used, one having few harmonics or very low in harmonic power, then beyond what the harmonics the tone generates is the harmonic contribution of whatever DUT, device under test, is contributing. This is a very overly simplistic explanation but it should give you an idea of what your seeing.

What I see in the third graph is the mains fundamental and it's harmonics. I think you've noticed the peaks are multiples of the 50Hz which mean they are harmonics of the 50hz.
The peaks at the upper end of the graph are something else.
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Old 11th May 2014, 11:27 PM   #4
EssB is online now EssB  United Kingdom
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The peak in the middle of the ARTA spectrum is a 1KHz signal.
Do you have ARTA's internal sine generator enabled ? Check that the 'Gen' dropdown box (just below left of the toolbar) is set to 'External'.
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Old 11th May 2014, 11:48 PM   #5
kouiky is offline kouiky  United States
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From what I have read on this site and the advice that I have received through PM from very respected members ( you know who you are), RMAA is a bit, well, sloppy. And, it's quite limited in its accuracy that is compounded by the audio interface, which is why lab grade equipment costs tens of thousands of dollars compared to hobby usb analyzers.

Seeing that the noise is harmonically related to 50Hz AC, there is a chance that the dongle is likely picking up some power supply noise. With low-cost audio analyzers, both the test bandwidth and upper limits are restricted to preserve some integrity. The noise seen in the upper limit may be related to the carrier tome that the close modulates at, or may be something simpler. I have a few ideas about it.
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Old 13th May 2014, 11:10 AM   #6
maxw is offline maxw  United Kingdom
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Thanks for the reply everyone.

In the third graph there is mains connected so that 50hz peak is probably that. But in the second graph there is also a 50hz peak when there is no mains connected to the DUT?

Quote:
Originally Posted by davada View Post
If you use sat a 1kHz tone then the fft will display this as a peak at 1kHz. Because the fundamental has the greatest power it will stand much taller than the other peaks. The other peaks are mostly harmonics of the fundamental tone at 2kHz, 3kHz, 4kHz and so on
Does that mean I can safely say that the those 2/3/4Khz peaks are not caused by the DUT? And ignore them?

Quote:
Originally Posted by EssB View Post
Do you have ARTA's internal sine generator enabled ? Check that the 'Gen' dropdown box (just below left of the toolbar) is set to 'External'.
Yes. If I set it to external then I need another device to generate the test signal, right?
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Old 13th May 2014, 12:00 PM   #7
SY is offline SY  United States
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Quote:
Originally Posted by maxw View Post

Does that mean I can safely say that the those 2/3/4Khz peaks are not caused by the DUT? And ignore them?
No and no. You have to do some work baselining your test, then get an idea of repeatability. Once you've got that nailed, you can separate DUT distortion from test distortion.
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Old 13th May 2014, 05:59 PM   #8
EssB is online now EssB  United Kingdom
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Quote:
Originally Posted by maxw View Post
... in the second graph there is also a 50hz peak when there is no mains connected to the DUT?
Still mains related pickup from somewhere. It can be difficult to get rid of. Try to keep DUT and connection leads away from powered items, or turn them off. You can get it down to a certain level but then you find it varies by time of day, the neighbour's washing machine, aliens tunneling, etc. Actually -120dB for a quick test hookup is not so bad, I've had worse.
ARTA has a high pass filter you can set to exclude most of the mains related junk from the THD measurements.

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If I set it to external then I need another device to generate the test signal, right?
Yes.

Last edited by EssB; 13th May 2014 at 06:00 PM. Reason: fix quotes
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Old 13th May 2014, 06:21 PM   #9
maxw is offline maxw  United Kingdom
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Quote:
Originally Posted by EssB View Post
Try to keep DUT and connection leads away from powered items, or turn them off. You can get it down to a certain level but then you find it varies by time of day, the neighbour's washing machine, aliens tunneling, etc. Actually -120dB for a quick test hookup is not so bad
Thanks! I've adjusted a few wires and got it down to -138dB now.

Quote:
Originally Posted by SY View Post
No and no. You have to do some work baselining your test, then get an idea of repeatability. Once you've got that nailed, you can separate DUT distortion from test distortion.
Yes, I've done loopback tests as a baseline and these peaks are there too
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Old 13th May 2014, 06:42 PM   #10
davada is offline davada  Canada
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I think you can benefit from reading this thread.

Low-distortion Audio-range Oscillator

The thread has been running for years and it's hugely long but well worth the read. Every means of measuring TH distortion have been discussed and a few enhancements for SC - FFT measurements evaluated. It's all there. It would be redundant to discuss it here.
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